1. Field of the Invention
The present disclosure generally relates to electronic circuits and, more specifically, to circuits comprising elements for generating one or several periodic signals.
An example of application of the present disclosure relates to integrated processors comprising a circuit for generating clock signals from a reference clock.
The present disclosure more specifically relates to the testing of the synchronization of a clock signal internally generated by an electronic circuit with respect to an internal or external reference clock.
2. Discussion of the Related Art
FIG. 1 schematically and partially shows an example of an electronic circuit 1 of the type to which the present disclosure may be applied. Such a circuit typically comprises multiple functions not shown (especially if it is a processor) and at least one circuit 11, for example, a phase-locked loop (PLL) type, for generating at least one internal clock signal CKint from a clock signal CLK, for example, an external clock signal.
A problem present in electronic circuits driven from clock or counting signals is to make sure that such signals effectively follow an expected value, that is, undergo no significant jitter with respect to the signal from which they are generated and more generally with respect to a reference signal.
FIG. 2 illustrates, in a timing diagram, an example of a jitter phenomenon of a signal CKint having a real period Tr which is slightly greater than an expected period TO. Offset Δt between the two periods generates a desynchronization of clock CKint which risks causing functional problems of the circuit.
Such a noise or phase jitter is more and more critical as the signal frequency increases, since the calculation results (for example, of a counter) rapidly drift with respect to an expected value.
A first method for testing the drift of a clock signal comprises sampling the signal generated by the circuit and measuring, with an external equipment, a possibly difference with respect to a reference signal. Such a method remains accessible for relatively low frequencies (a few hundreds of kilohertz) but can no longer be used for high frequencies (several megahertz, or even hundreds or thousands of megahertz) since the noise added by the external measurement elements and the connectors to the measurement point is not negligible.
A second method comprises integrating, in the circuit to be tested, a device providing information relative to the phase shift of the internal clock signal with respect to a reference clock, to enable external exploitation of the results.
FIG. 3 is a schematic block diagram illustrating a conventional example of such a device. In FIG. 3, only the measurement device integrated in circuit 1 has been shown. This device is formed of a shift register 21 based on D flip-flops having a data input receiving the output (for example, non inverted Q) of a sampling flip-flop 22. Data input D of flip-flop 22 receives internal clock signal CKint to be tested. Flip-flop 22 and shift register 21 receive, on their respective clock inputs, a signal CKref forming a reference clock of a frequency of the same order of magnitude as the signal to be measured. The n outputs Q1 to Qn of shift register 21 are provided outside of circuit 1 to a system 25 for exploiting the measurements (for example, a microcomputer). The function of shift register 21 is to parallelize the successive information on the clock edges and their possible offset, to provide information exploitable despite the high frequency.
A disadvantage of this second method is that it is necessary to load the data provided by shift register 21 into an external system to be able to exploit these data and determine a possible jitter between the two clock signals. In practice, such a device is limited to the product characterization and does not enable testing a possible clock jitter of a manufactured product, for example, in a final test before possible decision of discarding a product, or along the product lifetime.